A die casting machine is a molding machine in which an injection plunger provided in an injection device is driven and moved forward every shot to inject/fill a fixed amount of a molten metal material such as an Al alloy or an Mg alloy into a mold cavity to thereby form a product with a required shape. Similarly to an injection molding machine which injects/fills a plastic material into a mold cavity to form a product with a required shape, the die casting machine also injects/fills the molding material into the mold cavity through a low-speed injection step, a high-speed injection step and a intensification step (referred to as “holding pressure step” in the case of the injection molding machine). However, the die casting machine is characterized in that an injection speed in the high-speed injection step is about one digit higher than that in the injection molding machine. Therefore, in the background art, a hydraulic die casting machine which an injection plunger is driven by hydraulic pressure has been the mainstream.
However, the die casting machine provided with the hydraulic injection device can drive the injection plunger at high speed but has various problems that the scale of the plant and equipment is large, the energy efficiency is poor, the inside of the molding plant is contaminated with oil, and the working environment is bad. Therefore, in recent years, electric driven die casting machine without such a drawback have been proposed (e.g. see Patent Literature 1).
As the electric injection device of the die casting machine, the applicant of the present application has already proposed an electric injection device including: a first injection electric motor which is used for low-speed injection and intensification; a second injection electric motor which is used for high-speed injection; a first motive power transmission mechanism which transmits rotary motion of the first injection electric motor to a screw shaft of a ball screw mechanism; a second motive power transmission mechanism which transmits rotary motion of the second injection electric motor to the screw shaft; a first clutch mechanism which is provided in the first motive power transmission mechanism; a second clutch mechanism which is provided in the second motive power transmission mechanism; a nut body which is threadably mounted on the screw shaft; a linear motion body which holds the nut body; an injection plunger which is connected to the linear motion body at its one end; and a controller which controls start-up and stop of the first and second injection electric motors and connection and disconnection of the first and second clutch mechanisms; wherein: the controller stores start timings of a low-speed injection step, a high-speed injection step and an intensification step, starts up the second injection electric motor in a stop state before the start timing of the high-speed injection step, and changes over the second clutch mechanism from a disconnection state to a connection state in the start timing of the high-speed injection step or after the start-up timing of the second injection electric motor before the start timing of the high-speed injection step (see claim 1 of Patent Literature 1). According to the electric injection device stated in Patent Literature 1, the second injection electric motor for high-speed injection is started up in the stop state before the start timing of the high-speed injection step, and the second clutch mechanism for high-speed injection is changed over from the disconnection state to the connection state in or before the start timing of the high-speed injection step. Accordingly, the rotation speed of the second injection electric motor for high-speed injection can be enhanced in advance in the stage in which the second clutch mechanism has to be changed over from the disconnection state to the connection state in order to transmit the drive force of the second injection electric motor for high-speed injection to the screw shaft of the ball screw mechanism. Accordingly, it is possible to increase acceleration of the injection plunger driven through the ball screw mechanism and the linear motion body after the second clutch mechanism is changed over from the disconnection state to the connection state, so that it is possible to use injection motors comparatively low in output to execute a required injection step.